The present invention relates to the timing and amount of opening and closing of the combustion chamber valves of an internal combustion engine.
In the design of four-cycle internal combustion engines, it has been found desirable, when the engine is under load, for proper scavenging to provide a certain amount of overlap of the opening and closing of the intake and exhaust valves during each combustion chamber cycle. In engines of the type having a lobed cam timed for rotation with respect to the engine output shaft, the lobes of the cam are configured in shape and radial height so as to impart to an associated cam follower in the valve gear the appropriate motion and driving force for opening and closing of the intake and exhaust valves in accordance with a predetermined schedule of motion versus cam rotational position. It is known to design the cam lobes such that a certain amount of overlapping occurs between the closing of the exhaust valve upon termination of one combustion chamber cycle and the opening of the intake valve for beginning of the next complete cycle of the combustion chamber.
Previously it has been shown that the amount of opening and, to some degree, the timing of the combustion chamber valve may be altered by changing the length of the driving members in the engine valve gear between the cam and the valve while the engine is running. Such a known device is shown in Volume 50, No. 8 of the Society of Automotive Engineers Journal (TRANSACTIONS), August 1942 at page 69 in FIG. 28 thereof, in an article entitled "VALVE GEAR RESEARCH AS APPLIED TO DIESEL ENGINES" by Carl Voorhies. The technique of collapsing a hydraulic lash adjusting tappet is employed to change the length of the valve gear by selectively controlling flow of pressurized oil to the hydraulic tappet. The Voorhies device has been applied in combination with secondary lobes opposite the main lift lobe of a cam to alter the opening and timing of the exhaust valve of an engine for providing improved engine compression action braking in the motoring mode. See, for example, U.S. Pat. No. 3,786,792 of W. J. Pelizzoni et al. which teaches the use of a collapsible hydraulic tappet to render operative or inoperative a seondary cam profile useful for changing the exhaust valve timing to provide for engine braking.
It is known to use a two-stage latchable device (while the engine is running) for completely or partially disabling and re-enabling the intake and exhaust valves of an engine. See, for example, U.S. patent application Ser. No. 578,295 filed May 16, 1975 to Michael M. Walsh, and U.S. patent application Ser. No. 627,424 filed Nov. 30, 1975 to Martin W. Uitvlugt, which applications have been assigned to the assignee of the present invention. The latchable devices of Walsh and Uitvlugt utilize a rotary latch mechanism which, in one position, permits the lugs on a fulcrum member to move freely into recesses or slots when the fulcrum member is lifted by action of the cam lobe to thus absorb, by lost motion, the driving forces of the cam. The known latch has a second position in which the lugs register against other reactive members to transmit the driving forces of the cam and provide for normal opening and closing of the associated engine valve.
In an effort to reduce the fuel consumption and improve the smoothness of an engine running at idle or part-load, it has been proposed to reduce the normal opening and closing of the valves and to alter the timing such that overlap between the closing of the exhaust valve and the opening of the intake valve is reduced or eliminated. In order to accomplish this change in the valve lift and timing while the engine is running, it has been proposed to provide for selectively increasing the lost motion lash in the valve gear, thereby effectively reducing the lift height of the cam lobe and also reducing the period of time during which the valve is open. However, it has been found that, if the normal lost motion lash in an engine valve gear is suddenly increased substantially, that is, by an amount approximately six to ten times the normal lash, the members of the valve gear have acquired prohibitively high velocities at the instant the lash is taken up and the valve begins to move from its seat. With such increased lash present in the valve gear train, the valve is moving at a prohibitively high velocity when the valve contacts the valve seat and the driving forces are relaxed, lash recurs for any given cam profile suitable for opening and closing of valves with a normal low amount of lash.